Etchant with selectivity for doped silicon dioxide over undoped silicon dioxide and silicon nitride, processes which employ the etchant, and structures formed thereby

ABSTRACT

An etchant including C 2  H x  F y , where x is an integer from two to five, inclusive, where y is an integer from one to four, inclusive, and where x plus y equals six. The etchant etches doped silicon dioxide with selectivity over both undoped silicon dioxide and silicon nitride. Thus, undoped silicon dioxide and silicon nitride may be employed as etch stops in dry etch processes which utilize the C 2  H x  F y  -containing etchant. C 2  H x  F y  may be employed as either a primary etchant or as an additive to another etchant or etchant mixture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to processes for selectively etching dopedsilicon dioxide that overlies silicon nitride or undoped silicondioxide. Particularly, the process of the present invention includes anetchant mixture which includes the use of an ethane gas having thegeneral formula C₂ H_(x) F_(y), where x is an integer from two to five,inclusive, y is an integer from one to four, inclusive, and x plus yequals 6. The present invention also relates to etchant mixtures whichinclude a component having the general formula C₂ H_(x) F_(y), where xis an integer from two to five, inclusive, y is an integer from one tofour, inclusive, and x plus y equals 6.

2. Background of Related Art

The fabrication of multi-layered structures upon semiconductor devicestypically involves the patterning of doped silicon dioxide layers,including, without limitation, layers of phosphosilicate glass (PSG),borosilicate glass (BSG) and borophosphosilicate glass (BPSG). Suchmaterials are typically employed as passivation layers on semiconductordevices. Etching techniques are typically employed to pattern many typesof semiconductor device structures, including the formation of contactsthrough passivation layers. Etch stop layers are typically formed onunderlying structures in order to terminate the etch process once thedesired patterning of the passivation layer, or etch substrate, hasoccurred. Silicon nitride (Si₃ N₄) is typically utilized as an etch stopduring the patterning of silicon dioxide.

Typically, etching techniques include the depositing, masking andpatterning of protective layers, such as photoresists, which act astemplates, or protective masks, in order to define structures from apassivation layer by etching techniques. Wet etch or dry etch techniquesmay be employed to define semiconductor device structures from dopedsilicon dioxide passivation layers.

An exemplary wet etch process is disclosed in U.S. Pat. No. 5,300,463(the "'463 patent"), issued to David A. Cathey et al. The wet etchprocess of the '463 patent, which employs hydrofluoric acid (HF) as anetchant, is selective for doped silicon dioxide over undoped silicondioxide. Despite its specificity, that technique is somewhat undesirablefrom the standpoint that it suffers from many of the shortcomings thatare typically associated with wet etch processes. Specifically, thetechnique of the '463 patent is an isotropic etch. Consequently, thestructures defined thereby have different dimensions than those of thetarget area of the etch substrate that is exposed through the protectivemask. Moreover, as those of skill in the art are aware, since wet etchtechniques are typically isotropic, if the thickness of the film beingetched is approximately equivalent to the minimum desired patterndimension, the undercutting that is typically caused by isotropicetching becomes intolerable. Similarly, with the ever-decreasing size ofstructures that are carried on the active surfaces of semiconductordevices, etching must be very accurate and maintained within veryprecise tolerances in order to preserve the alignment of such minutestructures and to optimize the electrical characteristics of suchstructures. Such precision cannot be obtained while defining structureson semiconductor devices with many conventional wet etch processes.Thus, the lack of precision and isotropic nature of typical wet etchingprocesses are inconsistent with the overall goal of etch processes informing structures on state-of-the-art semiconductor devices:reproducing the features defined by the protective mask with a highdegree of fidelity.

In contrast, many dry etch techniques, including, without limitation,glow-discharge sputtering, ion milling, reactive ion etching (RIE),reactive ion beam etching (RIBE) and high-density plasma etching, arecapable of etching in a substantially anisotropic fashion, meaning thatthe target area of an etch substrate is etched primarily in asubstantially vertical direction relative to the exposed, or active,surface of the etch substrate. Thus, such dry etch techniques arecapable of defining structures with substantially upright sidewalls fromthe etch substrate. Consequently, such dry etch techniques are capableof accurately reproducing the features of a protective mask. Thus, dueto ever-decreasing dimensions of structures on semiconductor devices,dry etching is often desirable for defining structures uponsemiconductor device active surfaces.

Many techniques that employ plasmas to dry etch silicon dioxide layers,however, lack the specificity of comparable wet etch techniques sincefluorocarbons, such as CF₄ and CHF₃, are typically employed in plasmadry etches of silicon dioxide layers. The radio-frequency (RF) plasmasthat are typically utilized with many silicon dioxide dry etch processesgenerate activated species, such as fluoride ions and fluorine freeradicals, from such fluorocarbon etchants. While these activated speciesattack the silicon dioxide layer in order to etch the same, theactivated fluorine radicals and fluoride ions of many dry etchtechniques may also attack other materials, such as silicon and siliconnitride. Consequently, in addition to etching the desired layer, manydry etch techniques that employ plasmas also undesirably etch the etchstop layers and other structures of the semiconductor device that areexposed or which become exposed during the etching process.

Etch stop materials employed in dry etch techniques are typically etchedat a lower rate than the associated, usually underlying, etch substrate.Since the dry etchant etches the etch stop layer at a slower rate thanthe outer layer, the etch stop layer acts to protect structurestherebeneath from the dry etch process, even as the etch stop itself isbeing consumed.

Since the gate structures of many semiconductor devices include asilicon nitride (Si₃ N₄) cap, selectivity between silicon dioxide (SiO₂)and silicon nitride is desirable in order to etch contacts throughpassivation layers. Many of the so-called silicon dioxide-selectiveplasma dry etch techniques, however, have a SiO₂ to Si₃ N₄ selectivityratio, or etch rate of SiO₂ to etch rate of Si₃ N₄, of less than about3:1.

U.S. Pat. No. 5,286,344 (the "'344 patent"), issued to Guy Blalock etal. on Feb. 15, 1994, discloses a dry etch process which has much betterselectivity for silicon dioxide over silicon nitride than many otherconventional silicon dioxide dry etch techniques. Specifically, CH₂ F₂,which is employed as an additive to a primary etchant such as CF₄ orCHF₃, imparts the dry etchant mixture with improved selectivity forsilicon dioxide over silicon nitride. The high energy ions that arerequired to etch both silicon dioxide and silicon nitride act bydissociating a chemical bond at the respective oxide or nitride surface.The dissociation energy that is required to etch silicon nitride,however, is less than that required to etch silicon dioxide. The use ofCH₂ F₂ in the dry etchant causes polymer deposition on the siliconnitride surface that offsets the dissociation properties of siliconnitride relative to silicon dioxide relative to conventional dryetchants which lack additives such as CH₂ F₂. Thus, the etchant of the'344 patent etches silicon dioxide over an etch stop of silicon nitridewith a selectivity of greater than 30:1. As with other conventionalsilicon dioxide dry etch techniques, however, the only material that isdisclosed as a useful etch stop in the '344 patent is silicon nitride.Thus, the utility of the dry etch process that is disclosed in the '344patent is limited to defining semiconductor device structures whichinclude a silicon nitride dielectric layer, such as, for example,contacts over silicon nitride-capped gates. Moreover, the relative flowrates of each of the dry etchant components disclosed in the '344 patentare limited to narrow ranges in order to achieve the desired level ofselectivity. Similarly, many other conventional dry etch processesrequire the use of very specific dry etchant components. Thus, theprocess windows of many conventional dry etch systems are narrow.

Although silicon nitride is widely employed as an etch stop material,the use of silicon nitride etch stops is, however, somewhat undesirablefrom the standpoint that the deposition of silicon nitride upon asemiconductor device active surface by low pressure chemical vapordeposition (LPCVD) processes may also form a thick nitride layer on theback surface of the semiconductor device. Such thick nitride layers mustbe subsequently removed, which increases fabrication time and costs, aswell as the potential for damaging the semiconductor device during thefabrication thereof.

Morover, the fluorine radicals and fluoride ions that are generated byconventional dry etches which employ plasmas non-selectively attack, oretch, both doped and undoped silicon dioxide. Thus, such silicon dioxidedry etch techniques are incapable of distinguishing between doped andundoped silicon dioxide. Consequently, when conventional dry etchtechniques are employed, the use of alternatives to silicon nitride instate-of-the-art semiconductor devices is restricted.

Accordingly, the inventors have recognized a need for a selective dopedsilicon dioxide dry etch process for which both silicon nitride andundoped silicon dioxide act as etch stops, and etchants which arespecific for silicon dioxide over both undoped silicon dioxide andsilicon nitride. Etchant mixtures are also needed wherein relativeconcentrations of each of the components of such etchant mixtures may bevaried in order to facilitate the use of such mixtures in a broad rangeof doped silicon dioxide etching applications.

SUMMARY OF THE INVENTION

The present invention includes a dry etch process and etchants thataddress the foregoing needs and overcome the disadvantages manifested byconventional dry etch processes.

The etchants of the present invention include C₂ H_(x) F_(y), where x isan integer from two to five, inclusive, y is an integer from one tofour, inclusive, and x plus y equals 6. Specifically, the C₂ H_(x) F_(y)component of the present invention may be selected from the groupconsisting of C₂ H₂ F₄, C₂ H₃ F₃, C₂ H₄ F₂, and C₂ H₅ F. The C₂ H_(x)F_(y) component may be used as either a primary etchant or as acomponent of an etchant mixture. When employed as a primary etchant, C₂H_(x) F_(y) etches doped silicon dioxide at a slow rate relative to theetch rates of many conventional silicon dioxide dry etch techniques, butselectively etches doped silicon dioxide over undoped silicon dioxide.

When used as an additive to other silicon dioxide etchants, C₂ H_(x)F_(y) imparts the etchant mixture with selectivity for doped silicondioxide over undoped silicon dioxide, while permitting the doped silicondioxide etch to proceed at a comparable rate relative to manyconventional doped silicon dioxide dry etch techniques. The amount of C₂H_(x) F_(y) used in the etchant mixture may be varied, depending uponthe particular species of C₂ H_(x) F_(y) used, the desired level ofdoped to undoped silicon dioxide selectivity (i.e., selectivity ratio),the desired level of silicon dioxide to silicon nitride selectivity, thedesired etch rate, and other factors.

The dry etch process of the present invention employs an etchant of thepresent invention (i.e., an etchant which includes C₂ H_(x) F_(y)), andis selective for doped silicon dioxide over both undoped silicon dioxideand silicon nitride. Thus, the dry etch process of the present inventionmay be effectively employed for anisotropically etching a doped silicondioxide layer down to an underlying etch stop of either undoped silicondioxide or silicon nitride.

Other advantages of the present invention will become apparent to thoseof ordinary skill in the relevant art through a consideration of theappended drawings and the ensuing description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 are cross-sectional schematic representations whichillustrate the process of the present invention and exemplary structuresthat may be formed thereby.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention includes an etchant that is selective for dopedsilicon dioxide over both undoped silicon dioxide and silicon nitride.As those of skill in the art are aware, "doped" silicon dioxidetypically includes a dopant such as boron or phosphorus, whereas undopedsilicon dioxide is substantially free of dopants and other impurities.Exemplary types of doped silicon dioxide include, without limitation,borosilicate glass (BSG), phosphosilicate glass (PSG) andborophosphosilicate glass (BPSG). The present invention also includes adry etch process which utilizes the inventive etchant.

The doped silicon dioxide etchant of the present invention, which isalso merely referred to as an etchant for simplicity, includes an ethanecomponent of the general formula C₂ H_(x) F_(y), which is also referredto as the C₂ H_(x) F_(y) component or C₂ H_(x) F_(y) for simplicity,where x is an integer from two to five, inclusive, y is an integer fromone to four, inclusive, and x plus y equals 6. Specifically, the C₂H_(x) F_(y) component of the present invention is desirably selectedfrom the group consisting of C₂ H₂ F₄, C₂ H₃ F₃, C₂ H₄ F₂, and C₂ H₅ F.The doped silicon dioxide etchant may also include combinations ofvarious types of C₂ H_(x) F_(y).

As the C₂ H_(x) F_(y) component of a doped silicon dioxide etchant is RFactivated, the hydrogen ions and activated hydrogen species react withthe fluorine-containing ions and activated fluorine-containing species(e.g., F* and CF*), removing the activated fluorine-containing speciesfrom the surface of the wafer prior to the occurrence of any substantialamount of etching of an etch stop layer of either undoped silicondioxide or silicon nitride. The hydrogen content of the C₂ H_(x) F_(y)additives imparts etchants including the same with specificity for dopedsilicon dioxide over undoped silicon dioxide.

In a first embodiment of the doped silicon dioxide etchant of thepresent invention, C₂ H_(x) F_(y) is a primary etchant. When used as theprimary etchant, C₂ H_(x) F_(y) is selective for doped silicon dioxideover undoped silicon dioxide. Stated another way, C₂ H_(x) F_(y) etchesdoped silicon dioxide at a higher rate than it etches undoped silicondioxide. As the primary etchant, C₂ H_(x) F_(y) etches doped silicondioxide at a relatively slow rate compared to the etch rates of manyconventional silicon dioxide dry etchants. Thus, additives which willincrease the etch rate may be used in combination with C₂ H_(x) F_(y).Such additives include, but are not limited to, CF₄, CHF₃, and otherhalogenated carbon materials which have been used as primary etchants inconventional doped silicon dioxide dry etch techniques.

Similarly, additives that increase an etchant's selectivity for silicondioxide over silicon nitride (i.e., reduce the rate at which siliconnitride is etched) may also be used as additives to etchants whichinclude C₂ H_(x) F_(y) as the primary etchant. U.S. Pat. No. 5,286,344(the "'344 patent"), issued to Guy Blalock et al. on Feb. 15, 1994, thedisclosure of which is hereby incorporated by reference in its entirety,discloses some exemplary additives that may enhance the selectivity ofC₂ H_(x) F_(y) in this manner. The additives of the '344 patent arefluorocarbons in which the number of hydrogen atoms is equal to orgreater than the number of fluorine atoms, such as CH₂ F₂ and CH₃ F.

Other additives may also be used with silicon dioxide etchants thatinclude C₂ H_(x) F_(y) as the primary etchant in order to alter othercharacteristics of such etchants, including, without limitation, theselectivity of such etchants for doped silicon dioxide over undopedsilicon dioxide and the selectivity for certain types of doped silicondioxide over other types of doped silicon dioxide.

In another embodiment of the doped silicon dioxide etchant of thepresent invention, C₂ H_(x) F_(y) is employed as an additive to one ormore primary etchants. C₂ H_(x) F_(y) may be used as an additive toetchants which include a fluorocarbon primary etchant, such as CF₄,CHF₃, or other fluorocarbons which etch silicon dioxide at a higher ratethan they etch silicon nitride (i.e., are selective for silicon dioxideover silicon nitride). According to the '344 patent, CF₄ and CHF₃ areexemplary primary etchants with which C₂ H_(x) F_(y) may be utilized asan additive.

When used as an additive to a silicon dioxide etchant, such as CF₄ orCHF₃, C₂ H_(x) F_(y) imparts the silicon dioxide etchant withselectivity for doped silicon dioxide over undoped silicon dioxide whilepermitting the doped silicon dioxide etch to proceed at a substantiallynormal rate. The amount of C₂ H_(x) F_(y) that is used in an etchantmixture, relative to the amounts of other etchants and any carrier gas,may be varied in order to tailor the characteristics thereof and toachieve the desired etching results. The various characteristics of theetchant mixture which may be varied by altering the concentration of C₂H_(x) F_(y) in the mixture include, but are not limited to, selectivityfor doped silicon dioxide over undoped silicon dioxide, selectivity forsilicon dioxide over silicon nitride, and the doped silicon dioxide etchrate.

An exemplary dry etchant that is selective for doped silicon dioxideover both undoped silicon dioxide and silicon nitride includes about 40%of the additive C₂ H₂ F₄ (i.e., the C₂ H_(x) F_(y) component), about 30%of the primary etchant CHF₃, and about 30% of CH₂ F₂, an additive whichimproves the selectivity of the primary etchants for silicon dioxideover silicon nitride, each of the percentages based on the relative flowrates of each gas into the etcher.

Alternatively, the amounts of the C₂ H_(x) F_(y) component may be variedconsiderably. Etchants which include any amount of an additive of thegeneral formula C₂ H_(x) F_(y), where x is an integer from two to five,inclusive, where y is an integer from one to four, inclusive, and wherex plus y equals six, are within the scope of the present invention.Exemplary etchants may include five percent, ten percent, twentypercent, sixty five percent, or ninety percent of the C₂ H_(x) F_(y)additive or any combination of C₂ H_(x) F_(y) additives.

Similarly, it is also foreseen that C₂ H_(x) F_(y) may be employed as anadditive to silicon dioxide dry etchants which include other components.For example, C₂ H_(x) F_(y) could be used along with an etchant whichincludes either CF₄ or CHF₃ or both of them as primary etchants, and acarrier gas, such as argon or nitrogen. Alternatively, the C₂ H_(x)F_(y) -containing dry etchant may include one or more other additivesthat alter the various characteristics of the dry etchant, such as theetch rate, the degree of selectivity, and the type of selectivity Forexample, as disclosed in the '344 patent, the use of CH₂ F₂ as anadditive enhances the selectivity of the dry etchant for silicon dioxideover silicon nitride. Combinations of the additives of the generalformula C₂ H_(x) F_(y) may also be employed as components in a dopedsilicon dioxide dry etchant.

A preferred embodiment of the dry etch process of the present inventionemploys an etchant of the present invention (i.e., an etchant whichincludes C₂ H_(x) F_(y)), and is selective for doped silicon dioxideover both undoped silicon dioxide and silicon nitride. Thus, the dryetch process includes the etching of a doped silicon dioxide layer downto an etch stop of either undoped silicon dioxide or silicon nitride.

Referring to FIGS. 1 to 4, the etch process of the present invention,which utilizes the inventive etchant, is illustrated. FIG. 1 depicts anexemplary multi-layer structure 10, which is also referred to as asemiconductor device structure, that may be fabricated in part inaccordance with the process of the present invention. Multi-layerstructure 10 includes a semiconductor substrate 12 (e.g., a siliconwafer, silicon-on-insulator (SOI), silicon-on-sapphire (SOS),silicon-on-glass (SOG),etc.), a field oxide layer 14 that is disposed onan active surface 13 of the semiconductor substrate and an active deviceregion 16, polysilicon lines 18 disposed on the active device region,side wall spacers 20 positioned on each side of the polysilicon lines,an intermediate structural layer 22 disposed over each of the foregoingelements, and a passivation layer 24 disposed over the intermediatestructure layer. Passivation layer 24 is fabricated from doped silicondioxide, such as BPSG, PSG or BSG. Intermediate structural layer 22 maybe fabricated from either silicon nitride or undoped silicon dioxide.

FIG. 2 depicts masking of multi-layer structure 10 prior to defining astructure through passivation layer 24. A mask 26, which is alsoreferred to as a protective layer, is layered and patterned overpassivation layer 24. Mask 26 may be formed from a material such as aphotoresist, or photoimageable material. Exemplary positive photoresiststhat are useful as mask 26 may include a novolac resin, adiazonaphthaquinone, and a solvent, such as n-butyl acetate or xylene.Exemplary negative photoresists that are useful as mask 26 may include acyclized synthetic rubber resin, bis-arylazide, and an aromatic solvent.Such a mask 26 may be applied to, or coated onto, multi-layer structure10 and patterned by techniques that are known to those in the art, suchas spin coating and photomask processing and patterning techniques.Alternatively, mask 26 may comprise an aerosol spray pattern ofelectrostatically chargeable hardenable liquid material, such as apolymer, which is not etched or is etched at a much slower rate than theunderlying passivation layer 24. An exemplary method forspray-patterning such electrostatically chargeable hardenable liquidmaterials is described in U.S. Pat. No. 5,695,658 (the "'658 patent"),which issued to James J. Alwan on Dec. 9, 1997, the disclosure of whichis hereby incorporated by reference. Both photoresist materials(positive and negative) and non-photoimageable materials may be employedas mask 26 in accordance with the '658 patent. The utilization of masks26 which comprise other non-photoimageable materials and the processesfor applying and patterning them are also within the scope of the methodof the present invention. The patterning of mask 26 defines openings 28,which are also referred to as apertures or contact apertures,therethrough, through which predetermined structures will be defined inthe underlying passivation layer 24 during a subsequent etch step. Mask26 comprises a material that is resistant to the etchant of the presentinvention (i.e., the etchant does not etch mask 26 or etches the mask ata relatively slow rate compared to the rate at which the etch substrateis etched). Thus, the areas of passivation layer 24 which underlie mask26 are protected from the etchant during the subsequent etch step.

Turning now to FIG. 3, an etch step is depicted, wherein an etchant 30,which is introduced into an etch chamber (not shown) either with orwithout a carrier gas, attacks the areas of passivation layer 24 thatare exposed through openings 28 of mask 26. Dry etch processes that areknown to those of skill in the art, including, without limitation, highdensity plasma etching, reactive ion etching (RIE), magnetic ion etching(MIE), magnetically enhanced reactive ion etching (MERIE), plasmaetching (PE), point plasma etching, plasma enhanced reactive ion etching(PERIE), and electron cyclotron resonance (ECR), may be employed withthe etchant of the present invention and are within the scope of theprocess of the present invention. Etchant 30, which comprises a C₂ H_(x)F_(y) -containing etchant of the present invention, etches an aperturethrough passivation layer 24 in a substantially vertical fashion untilintermediate structural layer 22 is exposed. Intermediate structurallayer 22, which is fabricated from either undoped silicon dioxide orsilicon nitride, acts as an etch stop layer. Thus, etchant 30 etchesintermediate structural layer 22 at a slower rate than the rate at whichpassivation layer 24 is etched. After the exposed areas of passivationlayer 24 have been etched, mask 26 may be removed by processes that areknown in the art, such as washing or etching techniques.

FIG. 4 illustrates a contact opening 32, which is also referred to as acontact, that has been formed through passivation layer 24 by the etchprocess of the present invention. Contact opening 32 includes side walls34 that are substantially vertical relative to active surface 13 ofsemiconductor substrate 12. Contact openings 32 of the multi-layerstructure 10 expose at least a portion of the intermediate structurallayer 22 that lies above each of polysilicon lines 18, which may belogic circuits, such as word lines. Intermediate structural layer 22defines a cap 36 over each polysilicon line 18. Thus, cap 36 may befabricated from either undoped silicon dioxide or silicon nitride.

Although the foregoing description contains many specifics, these shouldnot be construed as limiting the scope of the present invention, butmerely as providing illustrations of some of the presently preferredembodiments. Similarly, other embodiments of the invention may bedevised which do not depart from the spirit or scope of the presentinvention. The scope of this invention is, therefore, indicated andlimited only by the appended claims and their legal equivalents, ratherthan by the foregoing description. All additions, deletions andmodifications to the invention as disclosed herein which fall within themeaning and scope of the claims are to be embraced within their scope.

What is claimed is:
 1. A process for selectively etching a doped silicondioxide structure, comprising;exposing the doped silicon dioxidestructure and at least a portion of an adjacent undoped silicon dioxidestructure to an etchant comprising C₂ H_(x) F_(y), where x is an integerfrom three to five, inclusive, y is an integer from one to three,inclusive, and x plus y equals six; and etching the doped silicondioxide structure with said etchant down to an etch stop comprised ofsaid adjacent undoped silicon dioxide structure without substantiallyetching said adjacent undoped silicon dioxide structure.
 2. The processof claim 1, further comprising disposing a protective layer over saiddoped silicon dioxide structure.
 3. The method of claim 2, furthercomprising patterning said protective layer to expose selected areas ofsaid doped silicon dioxide structure.
 4. The method of claim 2, whereinsaid protective layer comprises a photoimageable material.
 5. The methodof claim 4, further comprising patterning said protective layer byphotolithography.
 6. The method of claim 2, where said protective layercomprises a non-photoimageable material.
 7. The process of claim 1,wherein said etching is effected using a technique selected from thegroup comprising reactive ion etching, plasma etching, high densityplasma etching, point plasma etching, magnetic ion etching, magneticallyenhanced reactive ion etching, plasma enhanced reactive ion etching, andelectron cyclotron resonance.
 8. The process of claim 1, wherein saidetching comprises high density plasma etching or reactive ion etching.9. A method of patterning doped silicon dioxide, comprising:disposing amask material over the doped silicon dioxide; patterning said maskmaterial to expose selected regions of the doped silicon dioxide; anddry etching said exposed selected regions with an etchant comprising C₂H_(x) F_(y), where x is an integer from three to five, inclusive, wherey is an integer from one to three, inclusive, where x plus y equals six,said etchant being selective for the doped silicon dioxide over undopedsilicon dioxide and silicon nitride.
 10. The method of claim 9, furthercomprising providing an etch stop under the doped silicon dioxide andetching said exposed selected regions down to said etch stop.
 11. Themethod of claim 10, wherein said providing said etch stop comprisesproviding an undoped silicon dioxide etch stop.